Anytime a QB's best attributes are his "heart" and his "athleticism" rather than his ability to read defenses and accurately throw the damn football to the right receiver, I'd say you're looking at a sea of red flags. How many times do we have to see remarkable physical specimens like Randall Cunningham, Daunte Culpepper and Mike Vick fail as NFL QB's before we learn this lesson?
I wouldn't characterize Cunningham as a failure

Randall was 82-52 as a starter, with 3 seasons in which he accounted for 30+ TD's, not sure where he can be considered a failure. He didn't do it the prototypical way, but he got it done, and won a lot of games doing it. Vick as a failure is questionable as well, as he was 38-28-1 as a starter, and saw some success in the playoffs.
If the benchmarks are going to be wins, and numbers that impress, both have credentials. If the benchmark is going to be championships, well.. the list of failure pocket passers is going to be full of some really good QB's as well.
Flat out, you can't win a championship if your best offensive weapons are your QB's legs because he can't read defenses and run a complex passing offense. It won't happen. Yes, Cunningham and Vick have won games, and their athletic talent has been remarkable, but they ultimately failed - and yes, it's a failure - to win the big one because the offenses they ran were so limited. And a word about win totals - notable about those win differentials is that Cunningham's offense was never the focal point of the teams he played for except for one year . . . when he had a rookie WR on turf named Randy Moss who nobody knew how to cover, and that year alone contributed 14 wins to his +30 win differential. Otherwise he was on Eagles teams known for their defense, teams that could afford better than most to have such a limited offense, but even some of the best Eagles teams in the early 90's couldn't ultimately overcome Cunningham's limitations as a passer. Part of that's not Cunningham's fault - that's what you're going to expect when your best WR is Fred Barnett, and your best offensive weapon, outside of RC, was Keith Jackson.
Ditto Vick, who never had a WR who was any good, and whose best offensive weapon was Warrick Dunn. Vick's teams, too, relied upon solid defense and then took whatever Vick (and practically Vick alone) could offer them on offense.
Again, kudos to those guys for getting everything they could out of their talent - I'm not minimizing that. But you can't compete at the highest level when your offense is what amounts to the NFL version of a cult of personality. It takes a team to win championships in the NFL, and if you're going to be limited on offense then you'd better (in addition to an outstanding defense) at least be able to hand the ball off to a RB who can get 3-4 yards every time, as was the case with the Ravens in 2000 and the Bears in 1985.
That's what I'm talking about when I say those guys ultimately "failed".